We analysed age and sex-specific trends in the incidence of Hodgkin's lymphoma in Denmark, Sweden, Norway, and Finland during 1978-97. The incidence of Hodgkin's lymphoma decreased significantly in all age groups above 40 years; there was a significant increase in incidence among adolescents and young adults. This increase occurred primarily for Hodgkin's lymphoma of the nodular sclerosis subtype. These observations emphasise the need to identify risk factors for Hodgkin's lymphoma in the young.